A fuel cell is adapted to directly take out electric energy from electrodes disposed at both surfaces of an electrolyte by allowing fuel gas such as gaseous hydrogen and oxidant gas containing oxygen to electrochemically react with each other via the electrolyte. Particularly, a solid polymer fuel cell using a solid polymeric electrolyte comes to attention as an electric power source for an electric vehicle owing to a low operating temperature and easy handling. Specifically, a fuel cell vehicle is designed to mount thereon a hydrogen reservoir such as a high pressure hydrogen tank, a liquid hydrogen tank, or a hydrogen occlusion alloy tank, to send hydrogen supplied from the hydrogen reservoir and air containing oxygen to a fuel cell, in which the hydrogen and the oxygen react with each other, and then, to drive a motor connected to a drive wheel by electric energy taken out of the fuel cell. The fuel cell vehicle is an ultimately clean vehicle in which discharged substance is only water.
The fuel cell is a power generator capable of continuing power generation as long as fuel gas and oxidant gas are supplied. However, constituent elements such as an electrode catalyst and an electrolytic film are degraded in the course of usage, thereby deteriorating power generating performance.
Examples of a system for diagnosing the degradation of the fuel cell in accordance with the current-voltage characteristics of the fuel cell include a system disclosed in Patent Literature 1. This system derives an approximate expression of the relationship between a current and a voltage of the fuel cell, and then, diagnoses the degradation of the fuel cell using the approximate expression.
There has been known that an oxide film formed on a catalyst of an oxidant electrode in a fuel cell changes the current-voltage characteristics of a fuel cell, thereby decreasing a voltage. Since such a change in the characteristics (i.e., the decrease in voltage) is restored due to the reduction of the oxide film on the catalyst of the oxidant electrode during a halt of a fuel cell system, the current-voltage characteristics when the fuel cell system is brought to a halt is often remarkably different from those when the fuel cell system is restarted.
Patent Literature 1: Japanese Patent Application Laid-open No. 2000-357526 (page 5 and FIG. 2)